The present invention relates to an apparatus and a method for grinding an eyeglass lens such that it is fitted in an eyeglass frame.
Lens grinding apparatus are known that form a bevel or tapered edge on the periphery of an eyeglass lens such that it can be supportably fitted in the groove extending around an eyeglass frame. Apparatus of this type generally perform a bevelling operation with a cylindrical bevelling abrasive wheel having a V-shaped bevelling groove of a size that corresponds to the bevel to be formed on the periphery of the lens to be processed.
A problem with this apparatus using the bevelling abrasive wheel is that depending upon the angle of slope of the bevel's curve at a specific point during the bevelling operation and on the direction of the V groove in the abrasive wheel, the lens being processed is interfered with three-dimensionally by the bevelling abrasive wheel and the size of the bevel being formed becomes smaller than the desired value (not only in its width but also in its height). This problem could be solved by using a conical abrasive wheel but, a difficulty occurs if the bevel to be formed is trapezoidal or so low in height as to be flat in shape.
Another problem with the apparatus is that if the bevelling groove has only one size available, the size of the bevel to be formed cannot be adjusted in accordance with the size of the groove in the eyeglass frame that is variable with its constituent material and other factors. One way to deal with this problem is to use a bevelling abrasive wheel having different sizes of bevelling groove; however, the size of the bevel to be formed is not very flexible since it is determined by the size of the bevelling groove used; in addition, the overall layout of the abrasive wheel becomes complicated.
Further another problem arises with this eyeglass lens grinding apparatus. A bevel's apical locus is determined on the basis of the data for the configuration of the eyeglass frame and the position of the edge of the lens and processing data for bevel formation is calculated such that the center of the V groove in the bevelling abrasive wheel simply coincides with the determined bevel's apical locus.
The fact is the bevel's apical locus generally has a curvature, so if bevelling is performed on the basis of the processing data calculated in the manner just described above, the inclined processing surfaces of the bevelling abrasive wheel will interfere three-dimensionally with the bevel to be formed and the apex of the bevel actually produced is not as high as it should be. The interference is particularly significant when the curvature of the bevel's apical locus is strong and an unduly small bevel fails to ensure that the lens is snugly fitted in the eyeglass frame.